This invention relates to a discharge-type arrester and, more specifically, to a discharge-type arrester which comprises a pair of cylindrical main electrodes set in an insulating cylinder and an intermediate electrode coaxially surrounding a discharge gap between the main electrodes and coupled to the main electrodes through the discharge gaps. The invention can be applied to an arrester of a gas-filled gap type having at least one ignition conductor extending over part of the inner wall surface of the insulating cylinder along the longitudinal direction thereof.
As stated in, e.g., U.S. Pat. No. 4,187,526, the gas-filled gap type arrester has an advantage such that when one of the discharge gaps is activated, a common discharge chamber is ionized, thereby activating the other discharge gap without any time lag. Because they are high in insulating capability, free from leakage current, increased in discharge withstand current rating, and small in size, the arresters of this type are widely used as surge-protection arresters for, e.g., communication apparatuses.
However, the prior art gas-filled gap type arresters cannot fulfill all the requirements of the heavy duty standards, especially those in the United States.
An arrester stated in West German Pat. No. 3,100,924 is known as one of the gas-filled gap type arresters which comply with the U.S. heavy duty standards.
In the gas-filled gap type arrester stated in West German Pat. No. 3,100,924, a pair of main electrodes with inward steps thereon is formed into a double cylinder, and an intermediate electrode defining discharge gaps with the main electrodes is formed of a hollow cylinder which has a skirt-shaped section. Also, electrode activators formed of a metal oxide such as magnesium oxide are provided on the overlapping portions of the main and intermediate electrodes. Thus, the arrester can maintain satisfactory electrical properties against a surge current load and an AC current load during the period of its life.
In the prior art gas-filled gap type arrester, heat energy produced by the discharging operation is generally small for a lightning surge with a short discharge time, so that the arrester will not be heated to a high temperature. For an inductive surge from a power line requiring a relatively long arrester discharge time, however, the arrester is heated to a high temperature by substantial heat energy produced by the discharging operation.
Thus heated to a high temperature by the high-temperature heat energy, the arrester and its holder will be burned, possibly causing a serious accident.
In order to prevent such an accident and to securely ground the surge, one of the main electrodes of the prior art gas-filled gap type arrester is covered with a cup-shaped short bar by means of a disk fuse (low-melting-point alloy). The disk fuse is melted by discharged heat generated by the continuous discharge of the arrester, thereby moving the short bar toward the other main electrode to short-circuit the two main electrodes, that is, to establish the so-called short-circuit mode. Thus, the arrester is prevented from being burned by the heat attributed to the discharged current.
In the aforementioned conventional arrangement, however, the use of the cup-shaped short bar, covering the one electrode of the arrester, leads to an increased number of components and an increase in the overall size of the arrester, thus requiring a special case for the arrester. Possibly soiled by the melted fuse, moreover, the arrester case and other components need replacement.